Add a new field to dropt_option to store additional callback data.
This can be used to allow option handlers to store predefined values
to option variables (i.e., to offer functionality equivalent to
store_const from Python's argparse module).
Unfortunately, this warranted renaming the existing handler_data
member, so this might require adjusting any existing code that
initialized dropt_option members by name. Since I'm already breaking
API compatibility for 2.0, I might as well do this now.
Since C does not guarantee that integers may be converted to pointers
and back, for maximum flexibility, the new member should be a
uintptr_t instead of a void*. However, for compatibility with C89,
we can't use uintptr_t directly and must fall back to size_t. Alas.